Light-emitting diodes (LEDs) are optoelectronic devices made by placing a layer of light-emitting material between two electrodes. When a voltage potential is applied to the electrodes and current is injected through the material, visible light is emitted. Due to the high power efficiency, low cost of manufacture, durability and light weight of LEDs, LEDs are often used to create visual displays for portable electronic devices.
In recent years, intensive research and development have been conducted on light-emitting diodes (LEDs) for the realization of high efficiency color display, backlighting and illumination applications. One strategy is to use a metal anode to replace the typically used indium tin oxide (ITO) anode. Conventional organic light-emitting diodes (OLED) use indium tin oxide (ITO) as an anode material. The metal electrodes used for the anode have a number of advantages over the ITO and can have broad applications. For example, the metal anode may be suitable for the flexible substrates and the cost of production can be relatively low. Although power loss in a waveguide in the ITO anode can be eliminated, light may still be trapped in the form of waveguide modes in organic layers and surface plasmon polariton (SPP) modes associated with metal/organic interface within the OLED. The waveguide mode often refers to light trapped in or around the core of the guide, and the SPP modes are infrared or visible frequency electromagnetic waves trapped at metal-dielectric interfaces.
Various Bragg-gratings have been employed in the OLED devices to effectively extract light from the waveguide modes and SPP modes. Various methods have also been attempted to increase the outcoupling efficiency of OLEDs, including substrate modification techniques to recover the substrate modes, and nano-structured films to extract the ITO/organic modes and the SPP modes. However, most of these methods require complex techniques and expensive equipment, and the intensity of light can be enhanced only at special emission wavelengths in order to satisfy the Bragg condition. There is a need for OLEDs for which the enhancement of light intensity is not limited at particular emission wavelengths and which is easy to manufacture.